In the detection of various compounds through nuclear magnetic resonance (NMR) or electron spin resonance (ESR), one beginning condition is the imposition of a fairly uniform magnetic field at a specified flux level on the specimen of interest. In the laboratory, field shape and uniformity can be fairly easily controlled. There is an optimum which is achieved in the laboratory and difficult to achieve in the field. For instance, in the laboratory, the specimen is typically controlled in quantity (more than enough to provide a response) which quantity is located in an ampule or container of relatively small shape. This may not necessarily occur in the field; that is to say, the application of the present apparatus for detection of buried land mines, parcel bombs, letter bombs, bombs secreted in luggage intended for aircraft and the like may utilize a dispersed explosive. Dispersal of the explosive may place part of it outside the optimum and uniform magnetic field.
In laboratory conditions, it is possible to obtain a magnet system having poles which are appropriately shaped and positioned to form a uniform field. Again, practical applications may not be so kind. As an example, the present invention finds application in the inspection of mail parcels for bombs. To inspect packages, it may be necessary that the magnets be placed along a common face or plane of the package and are prevented from bracketing the package inasmuch as the package may be larger than the spacing between the magnetic poles. Positioning of the magnetic poles at the opposite ends of a specimen assists in providing a uniform field, particularly when the magnetic poles have a cross-sectional area which is relatively large in comparison with the specimen. The opposite may occur in a given situation where the poles are relatively small, located to the side of a specimen and dwarfed by the relative dimensions of the specimen so that the specimen, while being in a magnetic field, is certainly not in a uniform magnetic field or a field maintained at the optimum field intensity.
NMR and ESR techniques often require fairly tight control of field intensity. The field intensity must achieve some calculated optimum value to obtain the necessary resonance of interest. The present invention overcomes this handicap. As an example, it has been discovered that a field of 800.00 Gauss detects level crossing in the explosive RDX. The RDX may be concentrated or in an inert plastic dilutant; it is not critical to the operation of the NMR detection apparatus. The 800.0 Gauss value permits limited variation over a relatively narrow range. The phenomena of interest results from the interaction of the magnetic field resonance and an excitation signal from transmitter. The field strength and frequency of the transmitter are rigorously related. Accordingly, the margin for field variations is quite narrow and is not a broad band phenomena. Moreover, excessive flux density is equally a problem with deficient flux density.
One important factor in obtaining a good reading is that a time perturbation process occurs. Each interrogation or stimulation of the specimen at the requisite frequency of interrogation and magnetic flux density perturbs the specimen so that subsequent retesting requires a long wait; the length of time to permit the disturbance to subside is quite long and can vary widely with different compounds. This depends in large part on the makeup of the specimen, itself. The present invention provides repeated interrogations, but, as to a given nucleus in a specimen, the field inhomogeneities do not impact that nucleus unless and until the critical flux level range is achieved.
It is possible to provide a fixed frequency, variable magnetic flux NMR system. The reverse is also possible. The provision of field equipment suitable for testing of large volumes of letters, packages or luggage utilizes a U-shaped magnet which is typically inadequate in size and geometric configuration to provide a uniform field over the specimen. The specimen, itself, may be relatively small, sufficient to fill a few cubic centimeters. However, it may be located within a larger suitcase which is hundreds, perhaps thousands of times larger in volume. With this constraint in mind and in testing for typical explosives such as RDX or TNT, the spread of the magnetic field is about 2.0 to 50.0 Gauss for a range of samples and magnetic field strengths.
Again, referring to a commercial installation as opposed to laboratory equipment, it may be appropriate to position the magnet near a fixed locus of items to be inspected such as luggage. One alternative is to move the sample relative to the magnet. Another alternative is to move the magnet relative to the sample. In both cases, the net result is a variation in flux intensity which is a function of time and sample geometry. Ideally, the field should pass through the specimen of interest and the surrounding accoutrements so that the entire item of interest is swept by magnetic flux at the selected or optimum value. While certain sample portions may be swept by nonoptimum parts of the optimum magnet field (strength at an incorrect field intensity for excitation), it is possible to sum several sweeps of the specimen. This is particularly helpful in enhancing the signal-to-noise ratio (S/N) in that the coherent signal (from the optimum field portion) adds in a manner to reinforce, while the incoherent noise (from any source) is not additive in the same manner. Accordingly, while a given specimen of interest is stored within a large package or suitcase, the entirety of the package or suitcase can be swept and all responses summed coherently to provide an output signal which encodes the NMR response to the nuclei of interest.
This will apply whether the magnet forms a field with a small gradient or a high gradient. Indeed, it overcomes the drawback which occurs with high gradient fields, namely, where the NMR response is so small as a result of reduced effective sample volume that the signal of interest has heretofore been submerged in the noise. The signal-to-noise ratio limits available data interpretation techniques. Accordingly, the present invention provides signal summation via a digitizing and buffering approach which occurs prior to the detection process. Signal summation occurs within the circuit after the signal has been amplified, but not before it has been detected. Accordingly, it is working with the amplified but undetected RF signal. Positioning of the summation circuit ahead of nonlinear detection circuitry is, therefore, advantageous because it enhances the summed signal prior to detection.